Pattern formation during development of the embryonic cerebellum

The patterning of the embryonic cerebellum is vital to establish the elaborate zone and stripe architecture of the adult. This review considers early stages in cerebellar Purkinje cell patterning, from the organization of the ventricular zone to the development of Purkinje cell clusters—the precursors of the adult stripes.

Zones and stripes are important because cerebellar patterning influences all aspects of cerebellar organization and function. Here is not the place to expound this at length, but simply to note that the Purkinje cell map serves as a scaffold around which many other cerebellar structures are organized: 1. Afferent projections use Purkinje cells to target their terminal fields. 2. Interneurons are restricted at stripe boundaries and are thought to use Purkinje cell cues to establish their own topography. 3. Functional boundaries align with stripe boundaries (e.g., Chockkan and Hawkes, 1994;Chen et al., 1996;Hallem et al., 1999;Apps and Garwicz, 2005;Wadiche and Jahr, 2005).
Thus, Purkinje cell stripes lie at the heart of cerebellar structure, function, and pathology. How does this remarkable pattern develop? Where do Purkinje cell subtypes come from? How do they end up in stripes? Cerebellar pattern formation is conventionally divided into four broad stages (the timings refer to the mouse cerebellum: Figure 2): 1. The formation of the cerebellar ventricular zone (∼E7-E10 in mouse); 2. Purkinje cell birth in the ventricular zone and Purkinje cell subtype specification (∼E10-E13); 3. Purkinje cell migration from the SVZ and reorganization into a stereotyped embryonic cluster array (∼E14-E17); and 4. Purkinje cell cluster dispersal and refinement to form the adult stripes (∼E18-P20).
This review is focused on stages 2 and 3-Purkinje cell subtype specification and the early stages of pattern formation. We also focus this review on patterning of Purkinje cells-less is known of the development of patterns in either granule cells or inhibitory interneurons although many of these are thought to be secondary to the patterning of the Purkinje cells (e.g., Sotelo and Chédotal, 2005;Sillitoe et al., 2008Sillitoe et al., , 2010Chung et al., 2009a,b).

THE STRUCTURE OF THE VENTRICULAR ZONE
The cerebellar primordium arises from the rostral metencephalon between E8.5-E9.5 (e.g., Wassef and Joyner, 1997;Sillitoe and Joyner, 2007). It houses two distinct germinal matrices, the dorsal rhombic lip and the ventral ventricular zone (VZ) of FIGURE 2 | Pattern formation in the mouse cerebellum. Cerebellar pattern formation is divided here into three broad stages: Purkinje cell birth in the ventricular zone and Purkinje cell subtype specification (∼E10-E14); Purkinje cell reorganization into a stereotyped embryonic cluster array (∼E14-E18); and cluster dispersal and refinement to form the adult stripes (∼E18-P20  . Substantial Purkinje cell death occurs by apoptosis in the first postnatal week, and we hypothesize that this may play a role in refining the cerebellar map. The transformation of Purkinje cell clusters into stripes is triggered by Reelin binding to a Very Low Density Lipoprotein receptor/Apolipoprotein receptor 2 complex (Vldlr/Apoer2), which leads to tyrosine phosphorylation of the docking protein Dab1. These events are blocked by mutations of Reelin production (reeler), the Reelin receptors (Apoer2/Vldlr double null), or downstream signals (Dab1-/-).
Although the cerebellum contains a relatively small variety of neurons, the molecular machinery governing neuronal generation and/or subtype specification is still poorly understood. Genetic fate mapping shows that a Ptf1a-(pancreas transcription factor 1a-which encodes a bHLH transcription factor) expressing domain in the VZ gives rise to all Purkinje cells (Hoshino et al., 2005;Hoshino, 2006). In 2005, the characterization of a novel mutant mouse, cerebelless, which lacks the entire cerebellar cortex but survives into adulthood, was reported (Hoshino et al., 2005). The analysis of the phenotype, and the characterization of the underlying gene mutation, clarified that PTF1A is required for generating all cerebellar GABAergic compartment. ATOH1 and PTF1A participate in regionalizing the cerebellar neuroepithelium, and define two distinct areas, the VZ (Ptf1a) and the upper rhombic lip (Atoh1), which generate GABAergic and glutamatergic neurons, respectively (Hoshino et al., 2005;Pascual et al., 2007). In regard to GABAergic progenitors Purkinje cells are distinguished from interneurons by differential expression of E-cadherin in cycling progenitors (Mizuhara et al., 2010) and of the transcriptional corepressor Corl2 (Minaki et al., 2008) in post-mitotic precursors, while two other Ptf1a targets (Nephrin and Neph3) (Nishida et al., 2010) are expressed by all GABA progenitors. Moreover, the expression domains of three proneural genes (Ascl1, Neurog1, and Neurog2) overlap with that of Ptf1a in the VZ.
While many studies have investigated the roles played by ATOH1 in establishing the cerebellar glutamatergic lineage, fewer studies have explored GABAergic precursors born in the cerebellar VZ. In 2008, Zordan et al. published a systematic descriptive analysis of proneural gene expression at early stages of mouse cerebellar development (Figure 4). This established that at the onset of cerebellar neurogenesis (∼E11), the Ascl1 transcript becomes detectable in the VZ and presumptive NTZ. A similar distribution is observed at later stages, with the Ascl1 transcript occupying the entire thickness of the Ptf1a+ VZ all the way to its apical (ventricular) margin. Accordingly, the territories occupied by Ascl1 and Atoh1 are clearly complementary. Ascl1 remains confined to the VZ until E13.5. An additional study by Johnson and coworkers (Kim et al., 2011) described genetic fate mapping studies done by using two transgenic Ascl1-Cre lines, one of which expressed a tamoxifen-inducible Cre recombinase, CreERTM (Helms et al., 2005;Battiste et al., 2007) and two Creinducible reporter lines (Soriano, 1999;Srinivas et al., 2001). The evidence produced in this elegant lineage analysis study is in full agreement with Zordan et al. In particular, Ascl1+ progenitors are initially (E12.5) restricted to the cerebellar VZ and excluded both from the post-mitotic cerebellar transitory zone and from the rhombic lip migratory stream. The Ascl1+ and Atoh1+ progenitor domains are mutually exclusive, whereas a high degree of overlap exists between Ptf1a+ and Ascl1+ progenitors, suggesting that Ascl1 labels GABAergic neuronal progenitors. However, by E17.5 Ascl1+ progenitors are no longer confined to the VZ but are also found scattered throughout the cerebellar primordium.
Finally, a study by Wassef and co-workers (Grimaldi et al., 2009) further refined the analysis of the role of Ascl1 in cerebellar neurogenesis, incorporating the effects of Ascl1 gene disruption and overexpression. They established that Ascl1+ progenitors progressively delaminate out of the VZ to settle first in the prospective white matter, and then in the cerebellar cortex. By studying an Ascl1-GFP transgenic mouse, they demonstrated that Ascl1+ progenitors give rise to PAX2+ interneurons and OLIG2+ oligodendrocyte precursors, while glutamatergic neurons, astrocytes and Bergmann glial cells did not express GFP. In contrast, the loss of Ascl1 led to a dramatic reduction of PAX2+ and OLIG2+ precursors. No change was found in Purkinje cell development.
Finally, a gain-of-function approach by using in vivo electroporation of a GFP plasmid at E14.5, concluded that most Ascl1+ oligodendrocytes do not originate from the cerebellar VZ. In addition, an Ascl1 plasmid electroporated into the cerebellar Frontiers in Neuroanatomy www.frontiersin.org VZ led to an increased number of PAX2+ interneurons, fewer OLIG2+ oligodendrocyte precursors, and the complete absence of astroglia. This suggests that Ascl1 overexpression pushes progenitors toward a (PAX2+) interneuron fate and suppresses the astrocytic fate. Taken together, evidence suggests that Ascl1 contributes to GABAergic interneuron and cerebellar nuclear neuron generation, and to Purkinje cell development. However, it is not required for Purkinje cell specification.

POSSIBLE ROLES FOR NEUROGENINS IN THE DEVELOPMENT OF CEREBELLAR GABAergic NEURONS
Neurog1 and Neurog2 are expressed in the Ptf1a+ ventricular neuroepithelium. As shown by Zordan et al. (2008), the Neurog2 transcript is first observed around E11 in cerebellar nuclear neuron progenitors of the cerebellar primordium, whereas Neurog1 appears 1 day later, in a rostral region located between the isthmic organizer, labeled by Fgf8, and the territory marked by Ascl1. At E12.5, both Neurog1 (see also Salsano et al., 2007) and Neurog2 are present in the VZ but with a few differences in distribution: in the anterior cerebellum, Neurog1 is expressed at high levels in a region close to the midline, whereas Neurog2 is restricted to the lateral VZ. In posterior territories, the expression patterns overlap completely. Neurog1 and Neurog2 are adjacent to, and partially overlap with, post-mitotic domains labeled by Lhx1 and Lhx5, two genes that control Purkinje cell differentiation (Zhao et al., 2007). This suggests that Neurog1 and Neurog2 are expressed in progenitors that are undertaking the last cycle of cell division to become postmitotic Purkinje cell precursors. At E13.5 the differential anterior boundaries of Neurog1 and Neurog2 are maintained, although the transcript levels of both genes are down-regulated. The authors conclude that Neurog1 and Neurog2 are mainly expressed in the cerebellar germinal epithelium that gives rise to GABAergic progenitors, while they are completely absent from the rhombic lip, the source of all glutamatergic cerebellar progenitors. Moreover, their expression patterns are similar but not totally overlapping, suggesting that these two closely related genes may contribute to the diversity of cerebellar GABAergic neurons and, possibly, Purkinje cell subtypes.

Neurog1 IS EXPRESSED IN CEREBELLAR GABAergic INTERNEURON PROGENITORS
In 2009, Doughty and coworkers published a lineage analysis study that described the mature cerebellar neurons deriving from Neurog1+ cell fates in the developing mouse cerebellum (Lundell et al., 2009). They confirmed the findings of Zordan et al. (2008) and extended the analysis to late embryonic and postnatal cerebellar development. At E14-E20, Neurog1 is present in Ptf1a+ neurons, but it is excluded from the upper rhombic lip and external granular layer. Moreover, at P7, it co-localizes with Ptf1a and BrdU in the deep white matter. This suggests that Neurog1 is expressed in early GABAergic interneuron precursors that, shortly after birth, migrate from the white matter to reach their final destination in the cortex. By using two artificial chromosome (BAC)reporter mice they analyzed short-term and long-term Neurog1+ cell fates. Neurog1 is expressed in PAX2+ interneuron progenitors but it does not contribute to the GABAergic neuron lineage in the cerebellar nuclei (Vue et al., 2007). Surprisingly, they did not reveal any fluorescence in Purkinje cells. Furthermore, the authors bred Neurog1-Cre transgenic mice into the double reporter Z/EG line (Novak et al., 2000). Z/EG mice express a LacZ cassette under control of a CMV enhancer/chicken actin promoter (pCAGGS). In the presence of a Cre recombinase the lacZ cassette is excised, leading to the activation of the downstream EGFP gene. By using this approach, they revealed scattered GFP+ Purkinje cell neurons, mostly in the hemispheres. Surprisingly, this strategy failed to tag GABAergic interneurons, perhaps due to epigenetic/positional silencing of the reporter transgene or to low-level expression of the Neurog1-Cre transgene. A recent study confirmed the notion that, in the cerebellar primordium, the Neurog1+ lineage contributes to the Purkinje cell pool (Kim et al., 2011).
In summary, Neurog1 is expressed in progenitors giving rise to GABAergic interneurons of the cerebellar cortex and at least some Purkinje cells. However, it does not seem to contribute to the development of neurons of the cerebellar nuclei. While both neurogenin genes are expressed in the cerebellar VZ in presumptive GABAergic neurons, nothing can be inferred to date as regards their function(s). Do they affect cell type or subtype specification, or neuronal vs. glial commitment? And in either case, do they act redundantly with each other or with Ascl1? There seems to be some degree of selectivity, in that Neurog1 is expressed in only a share of GABAergic progenitors and, as predicted by Zordan et al. (2008), the broader expression domain exhibited by Neurog2, a direct PTF1A target gene (Henke et al., 2009), suggests that it may play a unique role in the development of GABAergic cerebellar nuclear neurons. No mechanism has been identified but a recent study of gene expression in the cerebellar primordium of E11.5 Neurog1 null mice suggests that Neurog1 and Pax6 may interact functionally in the activation of downstream targets (Dalgard et al., 2011).

PURKINJE CELL BIRTH AND THE FORMATION OF THE FIRST LAMINAR PHASE
Purkinje cells undergo terminal mitosis in the VZ between E10-E13 in the mouse (Miale and Sidman, 1961;Hashimoto and Mikoshiba, 2003;Namba et al., 2011: Figure 2). Birthdating studies, using incorporation of either adenovirus (Hashimoto and Mikoshiba, 2003) or bromodeoxyuridine (e.g., chick- Karam et al., 2000;mouse-Larouche and Hawkes, 2006), reveal a direct correlation between the birthdate of a Purkinje cell and its final mediolateral location, suggesting that Purkinje cells acquire positional information at or shortly after their terminal differentiation in the VZ. It is not known whether positional information and phenotype are specified at the same time. Post-mitotic Purkinje cells migrate dorsally out of the VZ, in part along radial glia processes (e.g., Morales and Hatten, 2006), and stack in the cerebellar anlage with the earliest-born Purkinje cells located most dorsally.

SPECIFICATION OF PURKINJE CELL SUBTYPES
In many areas of the CNS, the development of patterning is driven by neuronal activity (e.g., the retinotectal systemreviewed in Ruthazer and Cline, 2004), In contrast, Purkinje cell phenotype specification and stripe formation seem to be Frontiers in Neuroanatomy www.frontiersin.org activity-independent. Experiments both in vivo and in vitro suggest that the zebrin phenotype is specified early in development and is not influenced by subsequent interactions with cerebellar afferents, cerebellar neurons or glia (Leclerc et al., 1988;Wassef et al., 1990;Seil et al., 1995). For example, Purkinje cells in P0 cerebellar slice cultures express both zebrin+/− phenotypes, and blocking neuronal activity or depleting granule cells and glia did not change this (Seil et al., 1995). Next, deafferentation of the neonatal or adult cerebellum does not alter the fundamental zone and stripe architecture (zebrin I- Leclerc et al., 1988;HSP25-Armstrong et al., 2001). Similarly, cerebellar anlagen dissected from embryos at E12-E15 (prior to any contact with afferents- Paradies and Eisenman, 1993;Grishkat and Eisenman, 1995) and transplanted into either the anterior chamber of the eye or the neocortex of adult hosts (Wassef et al., 1990) had zebrin II+/− Purkinje cells in the mature grafts. Furthermore, zebrin+ Purkinje cells are more numerous in cultures of posterior cerebellum than anterior cerebellum, consistent with the expression pattern seen in vivo (Leclerc et al., 1988;Hawkes, unpublished data). Finally, several experiments have suggested a correlation between the time when a Purkinje cell is born and its final mediolateral position in the mature cerebellum (Hashimoto and Mikoshiba, 2003; suggesting that Purkinje cells' adult phenotypes are specified shortly after their birth in the VZ. What is known of Purkinje cell subtype specification? EBF2 is one of four members of a family of helix-loop-helix transcription factors highly conserved in evolution (reviewed in Dubois and Vincent, 2001;Liberg et al., 2002) that couple cell cycle exit to the onset of neuronal differentiation and migration (Garcia-Dominguez et al., 2003). Three of these (Ebf1-Ebf3) encode transcriptional activators expressed in cerebellar development (Figure 2, Croci et al., 2006). While no cerebellar defects have been described in Ebf1 or Ebf3 mutants, Ebf2 null mice feature a small cerebellum and apoptotic cell death of migrating and post-migratory Purkinje neurons. Of the Purkinje cells that survive, a major fraction is transdifferentiated into the zebrin II+ phenotype (Croci et al., 2006). An analysis of molecular markers of Purkinje cell subtypes revealed that EBF2 acts specifically to repress the zebrin II+ subtype, rather than to maintain the zebrin II-one . Interestingly, unlike EBF3, EBF2 is sensitive to Notch-mediated repression in Xenopus neurulae (Pozzoli et al., 2001), both at the transcriptional and at the functional level, suggesting that Notch signaling may affect the determination of Purkinje cell subtypes by modulating EBF2, for instance by switching off Ebf2 expression in the early born Purkinje cell population. Consistent with this hypothesis, genetic tagging of EBF2+ cells by using an Ebf2-Cre transgene reveals that all Purkinje cells are initially Ebf2+ (Consalez, unpublished). Conditional overexpression experiments are now required to elucidate the crucial stages at which EBF2 affects Purkinje cell subtype specification in pre-and postnatal development.

THE MIGRATION OF PURKINJE CELLS TO FORM EMBRYONIC CLUSTERS
Post-mitotic Purkinje cells migrate from the VZ and stack in a layer with the earliest-born located dorsally (and becoming zebrin II+) and the youngest ventrally (and becoming EBF2+: Figure 2). Subsequently the layer undergoes a quite complicated reorganization (Miyata et al., 2010), possibly involving cell-signaling molecules including cadherins (e.g., Neudert and Redies, 2008;Redies et al., 2011) and ephrins (e.g., Karam et al., 2000;Sentürk et al., 2011), to yield a stereotyped array of embryonic Purkinje cell clusters with multiple molecular phenotypes. Grafts of dissociated Purkinje cells also organize into discrete zebrin+/− compartments, pointing to cell-cell adhesion molecules as possible organizers (Rouse and Sotelo, 1990). Each Purkinje cell cluster is separated from its neighbors by narrow gaps ("raphes"), later filled by migrating granule cells. In general terms, expression data show up to 10 embryonic clusters, arrayed symmetrically from medial to lateral on each side of the midline.

CLUSTER ARCHITECTURE IN THE EMBRYONIC CEREBELLUM
The reorganization of the early lamina results in a highly reproducible array of Purkinje cell clusters that can be distinguished through the differential expression of numerous molecules. The expression profiles are of three kinds. First, there are molecules that are selectively expressed at some time during embryogenesis but subsequently disappear and are not expressed in the adult (e.g., neurogranin- . Secondly, some molecules are selectively expressed during embryogenesis but are expressed by all Purkinje cells in the adult (e.g., calbindin- Wassef et al., 1985). Thirdly, a few molecules are selectively expressed by Purkinje cell subsets both in the embryo and the adult (e.g., PLCβ4-Marzban et al., 2007); and finally, in some cases expression reveals one pattern in the neonate and a different one in the adult (e.g., HSP25-Armstrong et al., 2001).
Examples of embryonic cluster markers include: Calbindin (Calb1) is a major calcium binding protein that acts as a buffer to protect neurons from neurotoxicity. At around P0, calbindin expression defines three Purkinje cell clusters on each side of the midline (Wassef et al., 1985;. In the adult cerebellum, calbindin is expressed uniformly by all Purkinje cells Phospholipase Cβ4 (PLCβ4) is a signal transducer. PLCβ4immunoreactive Purkinje cells in neonatal mice reveal two, three, and four parasagittal domains in the AZ, CZ, and PZ respectively. Later, differential expression of PLCβ4 in the AZ and PZ reveal a striped pattern in the mature cerebellar cortex . Engrailed-2 (En2) is a transcription factor important for the differentiation of Purkinje cells and the adult ("late-onset") banding pattern. En2 expression, labels three distinct cluster domains at E17.5 but expression is suppressed in Purkinje cells after birth (Millen et al., 1995). Cadherins mediate cell adhesion and play fundamental roles in the growth and development of many cells. Purkinje cell clusters express multiple members of this superfamily and some, such as cdh8, pch7, and pcdh10, are expressed differentially. For example, in the mouse cerebellar cortex at P3 cdh8-immunoreactive Purkinje cells form two parasagittal clusters each side of the midline, there are three pch7+ clusters, and a single pcdh10 cluster. In some cases, such as Frontiers in Neuroanatomy www.frontiersin.org pcdh10, expression is maintained and the cerebellum displays a striped pattern in the adult cortex (Redies et al., 2011). Heat shock protein 25 (HSP25) is involved in stress resistance by acting as a chaperone that binds to and stabilizes the active conformations of other proteins. At P1, the anterior lobe of the mouse cerebellum presents two distinct pairs of clusters of HSP25+ Purkinje cells arrayed symmetrically about the midline in the AZ and PZ . During later postnatal development HSP25 is transiently expressed by all Purkinje cells, until in the adult expression becomes restricted to a quite different pattern of stripes (in the CZ and NZ: Armstrong et al., 2000. Purkinje cell protein 2-lacZ transgene (L7/pcp2-lacZ): pcp2 is a G-protein regulator that is widely expressed in Purkinje cells. Around P0, the differential expression pattern of an L7/pcp2-lacZ transgene reveals three distinct compartments in each hemicerebellum (Oberdick et al., 1993;Ozol et al., 1999).
In the adult cerebellum, L7/pcp2-lacZ expression remains in stripes of Purkinje cells, especially in the AZ and PZ (Ozol et al., 1999). Synaptotagmin IV (Syt IV) is involved in early neural differentiation including axonal growth and the formation and consolidation of synapses. At P0, Syt IV is weakly expressed in select Purkinje cell clusters. From P15 onward, all Purkinje cells are Syt IV+ (Berton et al., 1997). Neurogranin (Nrgn) is a neural calmodulin-binding protein thought to play an important role in synaptic transmission and neuronal plasticity. At E17, Nrgn expression in the AZ and PZ reveals three and four parasagittal pairs of neurogranin expressing Purkinje cell clusters respectively . These disappear in the adult.

Inositol 1,4,5-trisphosphate (IP3) receptor-lacZ transgene
(IP3Rnls-lacZ): IP3R is a ligand-gated calcium channel, which is highly expressed in Purkinje cells. From E15 to P0, two clusters of Purkinje cells are selectively labeled on either side of the midline. Transgene expression continues to reveal heterogeneous Purkinje cells stripes in both the vermis and the hemisphere in the adult (Furutama et al., 2010). Wnt7b is a signaling molecule involved in CNS development. At E18, Wnt7b expression reveals three mediolateral Purkinje cells clusters: Wnt7b expression is shut down in the adult (Hashimoto and Mikoshiba, 2003).

Olfactory marker protein-lacZ transgene (OMP-lacZ):
The pattern of expression of an OMP-lacZ fusion gene (from E14.5 to P0) demonstrates three clusters on each side of the cerebellar midline. In the adult cerebellum, the pattern of transgene expression continues to reveal a striped pattern, restricted to the posterior lobe (Nunzi et al., 1999). Early B-cell factor 2 (EBF2): Sections through the mouse cerebellum show EBF2 is expressed shortly after birth in multiple stripes and wholemount staining of adult cerebellum shows EBF2-lacZ is expressed in stripes restricted to the AZ and CZ, equivalent to the distribution of the zebrin II-Purkinje cell subset (Croci et al., 2006). Cyclic GMP-dependent protein kinase (cGK) is implicated in multiple biological functions, including axon guidance, synaptic plasticity and learning. Transverse sections of rat cerebellum taken between E17-P3 reveal two discrete clusters of Purkinje cells that are immunoreactive for cGK. In adults, all Purkinje cell express cGK (De Camilli et al., 1984;Wassef et al., 1985). Ephrin type-A receptor 4 (EphA4): is a tyrosine kinase receptor. Transverse sections through the mouse cerebellum at E18.5 display four distinct EphA4+ clusters of Purkinje cells (Hashimoto and Mikoshiba, 2003). In the adult cerebellum, EphA4 expression appears homogenous, except perhaps for some areas of the hemispheres (Karam et al., 2000). PEP-19: is a developmentally regulated polypeptide that modulates calmodulin function. The expression pattern demonstrates three clusters on each side of the cerebellar midline (Herrup and Kuemerle, 1997). PEP19 expressed in all Purkinje cells of the adult rat cerebellum (Mugnaini et al., 1987).
While these, and other, markers reveal embryonic cerebellar complexity, the relationships between the various topographic maps are poorly understood. A speculative but most useful synthesis is presented in Herrup and Kuemerle (1997).
In the embryo the cluster is ∼10 Purkinje cells deep. As the clusters disperse into adult stripes the Purkinje cells spread to form a monolayer. Because dispersal occurs primarily in the anteroposterior plane, as the lobules of the cerebellum form, the rostrocaudal length of the cerebellum increases ∼25-fold while the width of the vermis increases only ∼1.5-fold (Gallagher et al., Frontiers in Neuroanatomy www.frontiersin.org 1998). As a result the clusters string out into long parasagittal stripes. Most adult stripe markers are first expressed during this period. A few already show more-or-less adult patterns of restriction by around P5 (e.g., PLCβ4-Marzban et al., 2007) but most-including zebrin II (Lannoo et al., 1991a,b)-are first expressed at around this time but go through a "global expression" phase in which they are expressed by all Purkinje cells (e.g., HSP25-Armstrong et al., 2001;zebrin II-Lannoo et al., 1991a;Rivkin and Herrup, 2003;OMP-lacZ-Nunzi et al., 1999) before they are selectively down-regulated and the stripe architecture matures by P20. What is the topographical relationship between the embryonic clusters and the adult stripes? By E18, numerous Purkinje cell molecular markers show restriction to subsets of clusters. The accumulated data from expression mapping of single markers suggest the possibility of a straightforward embryonic architecture: all known early markers appear to be restricted to the same schema with no more than ∼10 clusters on each side of the midline. So why are there many more adult Purkinje cell stripes than there are embryonic clusters (several hundred stripes vs. a few dozen clusters)? One explanation is that clusters are much more complex than is generally appreciated. By this view, the elaborate adult topography arises because each "simple" embryonic cluster in fact comprises multiple sub-clusters. In some cases there may be internal partitions (e.g., a medial vs. a lateral component of a cluster, each becoming a separate stripe in the adult); in other cases, Purkinje cells of different phenotypes may be intermingled within a cluster but segregate into separate stripes as the cluster transforms into stripes. Alternatively, each embryonic cluster may be homogeneous and additional complexity introduced into the adult map because individual clusters disperse into multiple stripes of the same adult phenotype. One previous study supports "complex dispersal"-in the weaver mouse two clusters fail to disperse and three adult stripes are missing, all of the zebrin II+/HSP25+ phenotype . Sillitoe et al. (2009) reveal a similar story by using a pcp2-CreER-IRES-hAP transgene to tag three bilateral clusters on approximately E15 and show they yielded zebrin II+ Purkinje cells of nine adult stripes. On the other hand, in some cases several embryonic clusters merge to form a single stripe. The clearest example are the zebrin II-/PLCβ4+ stripes in the vermis of the AZ, which are seen to be subdivided into triplets by the pattern of mossy fiber innervation (Ji and Hawkes, 1994) and arise from the fusion of three perinatal PLCβ4+ clusters .

ROLE OF PROGRAMMED PURKINJE CELL DEATH?
Finally, during the perinatal period it is clear that significant Purkinje cell death occurs (reviewed in Vogel, 2002: Figure 2). Does this play a role in the sculpting of cerebellar topography? Two complementary hypotheses can be considered. First, studies of naturally occurring cell death in the cerebellum have identified a spatial organization to Purkinje cell apoptosis ("hot spots": Jankowski et al., 2009) that correlates with stripe boundaries in the adult, and propose the interesting hypothesis that cell death may sharpen the acellular raphes between clusters. In addition, naturally occurring cell death could be an error-correction mechanism. A striking feature of adult cerebellar topography is its high reproducibility between individuals and its attendant low error rate (e.g., zebrin II+ Purkinje cells are very rarely seen in zebrin II-stripes). If stripes derive from clusters, and stripes have no errors, then either clusters have no errors (and migration from the VZ to the clusters is perfect) or errors that occur during cluster formation are subsequently eliminated. In this context it is interesting that many Purkinje cells-perhaps as many as a third-undergo cell death by apoptosis during the perinatal period (Dusart et al., 2006;Jankowski et al., 2009). This suggests the hypothesis that perinatal apoptosis might eliminate Purkinje cells that wind up in the wrong embryonic cluster (possibly via a local insulin-like growth factor 1 pathway- Croci et al., 2011; see also Jung et al., 2008). Purkinje cell ectopia is not lethal per se: for example, clusters that fail to disperse normally do not die [e.g., reeler (Goffinet, 1983;Edwards et al., 1994), Vldlr-/-:Apoer2-/- (Larouche et al., 2008), Dab1-/- ], and Purkinje cells located ectopically in the molecular or granular layers survive indefinitely (e.g., Rouse and Sotelo, 1990;Carletti et al., 2008). Rather, one might evoke a community effect (à la Yang et al., 2002), such that being in the wrong cluster during development leads to apoptosis.

PURKINJE CELL ARCHITECTURE AS A SCAFFOLD FOR CEREBELLAR TOPOGRAPHY
It is generally believed that the Purkinje cell architecture is the scaffolding around which many other cerebellar components are organized (e.g., reviewed in Sotelo and Wassef, 1991). For example, both climbing fiber and mossy fiber afferents terminate in the cerebellum as stripes that align with those revealed by stripe antigens (e.g., climbing fibers- Chédotal et al., 1997;Sotelo and Chédotal, 2005; mossy fibers- Sotelo and Wassef, 1991;Ji and Hawkes, 1995;Armstrong et al., 2009). In some cases, this can be very precise: for example somatostatin-immunoreactive mossy fibers terminate precisely beneath a very small subset of Purkinje cell stripes (∼2%) that constitutively express HSP25 (Armstrong et al., 2009). The alignment of the afferent and Purkinje cell (= efferent) maps is established early in cerebellar development where the earliest mossy fiber topography is seen as transient, possibly functional, contacts between mossy fibers and Purkinje cells (e.g., Mason and Gregory, 1984;Takeda and Maekawa, 1989) in specific embryonic clusters (Grishkat and Eisenman, 1995;Paradies et al., 1996). When the embryonic clusters disperse into stripes the afferents appear to move with them, thereby retaining the topographic relationship with a particular Purkinje cell subset. During postnatal development, mossy fibers move to the granular layer but stay aligned with the Purkinje cell stripe (e.g., Arsénio Nunes and Sotelo, 1985;Ji and Hawkes, 1995). A similar mechanism seems to serve to guide cerebellar interneurons to their specific stripe locations (unipolar brush cells- Chung et al., 2009a,b;Golgi cells-Sillitoe et al., 2008) and boundaries between stripes restrict the mediolateral spread of Golgi cell dendritic arbors (Sillitoe et al., 2008).
Finally, a different, and not well-understood, process also restricts granule cell dispersal. There are several subclasses of granule cell based both on gene expression (e.g., reviewed in Hawkes and Eisenman, 1997;Ozol and Hawkes, 1997) Frontiers in Neuroanatomy www.frontiersin.org and lineage . Transverse boundaries that separate granule cell lineages align with transverse zone boundaries identified in the Purkinje cell scaffold. How this comes about is not understood.